This invention relates to telephone line circuits and, more particularly, to line circuits for supplying battery feed to one or more remote telephone sets.
Most line circuits for supplying the DC current required by a telephone set's carbon microphone employ a line transformer. The transformer permits a balanced line circuit to be connected to an unbalanced switching path, it provides impedance transformation, and, if properly connected, it prevents longitudinal voltages appearing on the balanced line side from being coupled to its unbalanced side.
The line windings of the transformer either may be connected to carry the DC line current needed by the remote carbon microphone or they may be isolated from the DC current by means of a blocking capacitor. In the latter case, shunt feed inductors must usually be employed to prevent the low-impedance battery supply from short circuiting the AC components of the speech signal.
Heretofore, it has been suggested that the bulky current feed inductors could be replaced by a pair of opposite conductivity-type transistors biased for constant current operation to prevent excessive current on short loops and to provide adequate current on long loops. Examples of this type of shunt feed may be found in J. K. Livingstone U.S. Pat. No. 3,035,122, issued May 15, 1962, and in S. Orbach U.S. Pat. No. 3,955,052, issued May 4, 1976. It is to be noted, however, that these shunt feed arrangements require the use of a high quality blocking capacitor having low leakage and adequate capacitance to provide good AC coupling of speech signals to the transformer winding. In addition, the dielectric material of the blocking capacitor should be able to withstand lightning voltage surges of several hundred volts that may occur if one of the pair of carbon block protectors develops high impedance to ground with age.
On the other hand, in the conventional series feed arrangement, the two split primary windings of the line transformer respectively carry the DC microphone current to the tip and ring conductors of the telephone line. Unfortunately, the need to carry the DC current requires that more "iron" be employed in the transformer core to avoid core saturation. The increased size and weight of such construction detracts from the utility of series feed arrangements in the newer, lightweight and compact styles of construction which have come into vogue in the telephone industry with the advent of printed wiring boards and the expanding usage of integrated circuit technology.
In the copending application of J. F. O'Neill, Ser. No. 711,809, filed of even date herewith, it is shown that the use of a third winding poled oppositely with respect to the tip and ring windings can be used to balance-out their net DC magnetization. The O'Neill application shows that the AC components of the speech signals may be bypassed by a capacitor if the third winding is driven from the collector circuit of a properly biased transistor. The high AC impedance of the transistor collector prevents the third winding from being short circuited by the capacitor at voice frequencies. While it is important that the bypass capacitor be sufficiently large to offer a low impedance path to the "wire components" of speech signals, a capacitor of such size will prevent dial pulses from being easily detected. In the copending application of J. E. Dalley Ser. No. 711,811, filed of even date herewith, it is shown that a negative feedback operational amplifier may be connected in such a manner as to multiply the effective capacitance exhibited by the bypass capacitor at audio frequencies but to saturate in response to step function inputs occasioned by the cessation of line current during dial pulsing so that the bypass capacitor will then exhibit only its intrinsic capacitance and, thereby, not inhibit voltage changes from rising to magnitudes that may be easily detected. While both the Dalley and O'Neill circuits are advantageous from the standpoint of allowing reductions to be obtained in the weight and size of a line transformer, the particular electronic circuitry therein disclosed may not be conveniently integrable. Accordingly, it would be advantageous to obtain a line circuit having a small transformer and which in addition is easily integrable.